Self-propelled or powered vehicles, such as power wheelchairs, have vastly improved the mobility/transportability of the disabled and/or handicapped. Whereas in the past, disabled/handicapped individuals were nearly entirely reliant upon the assistance of others for transportation, the Americans with Disabilities Act (ADA) of June 1990 has effected sweeping changes to provide equal access and freedom of movement/mobility for disabled individuals. Notably, various structural changes have been mandated to the construction of homes, offices, entrances, sidewalks, and even parkway/river crossing, e.g., bridges, to include enlarged entrances, powered doorways, entrance ramps, curb ramps, etc., to ease mobility for disabled persons in and around society.
Along with these societal changes, has brought an opportunity to offer better, more agile, longer-running and/or more stable powered wheelchairs to take full advantage of the new freedoms imbued by the ADA. More specifically, various technologies, initially developed for the automobile and aircraft industries, are being successfully applied to powered wheelchairs to enhance the ease of control, improve stability, and/or reduce wheelchair weight and bulk. For example, sidearm controllers, i.e., multi-axis joysticks, employed in high technology VTOL and fighter aircraft, are being utilized for controlling the speed and direction of powered wheelchairs. Innovations made in the design of automobile suspension systems, e.g., active suspension systems, which vary spring stiffness to vary ride efficacy, have also been adapted to wheelchairs to improve and stabilize powered wheelchairs. Other examples include the use of high-strength fiber reinforced composites, e.g. graphite, fiberglass, etc. to improve the strength of the wheelchair frame while reducing weight and bulk.
One particular system which has gained widespread popularity/acceptance is mid-wheel drive powered wheelchairs, and more particularly, such powered wheelchairs with anti-tip systems. Mid-wheel powered wheelchairs are designed to position the drive wheels, i.e., the rotational axes thereof, slightly forward of the occupant's Center Of Gravity (COG) to provide enhanced mobility and maneuverability. Anti-tip systems provide enhanced stability of the wheelchair about its pitch axis and, in some of the more sophisticated anti-tip designs, improve the obstacle or curb-climbing ability of the wheelchair. Such mid-wheel powered wheelchairs and/or powered wheelchairs having anti-tip systems are disclosed in Schaffner et al. U.S. Pat. Nos. 5,944,131 & 6,129,165, both issued and assigned to Pride Mobility Products Corporation located in Exeter, Pa.
While such wheelchair designs have vastly improved the capability and stability of powered wheelchairs, designers thereof are continually being challenged to examine and improve wheelchair design and construction. For example, the Schaffner '131 patent discloses a mid-wheel drive wheelchair having a passive anti-tip system. A brief examination thereof reveals that two separate and distinct suspension struts are employed for mounting (i) the drive wheel/drive train assembly to the main structural frame of the wheelchair, and (ii) an anti-tip wheel to a forward portion of the main structural frame. As such, passive anti-tip systems typically necessitate the use of two independent spring-strut assemblies thus increasing mechanical complexity, maintenance requirements, cost (i.e., the cost of two spring-strut assemblies), and weight.
The Schaffner '165 patent discloses a mid-wheel drive powered wheelchair having an anti-tip system which is “active” in contrast to the passive system discussed previously and disclosed in the '131 patent. Such anti-tip systems are responsive to accelerations or decelerations of the wheelchair to actively vary the position of the anti-tip wheels, thereby improving the wheelchair's ability to climb curbs or overcome obstacles. More specifically, the active anti-tip system mechanically couples the suspension system of the anti-tip wheel to the drive-train assembly such that the anti-tip wheels displace upwardly or downwardly as a function of the magnitude of torque applied to the drive train assembly.
The systems are mechanically coupled by a longitudinal suspension arm pivotally mounted to the main structural frame. To one end of the suspension arm is mounted a drive-train assembly, and, to the other end, an anti-tip wheel. To better visualize the arrangement, it is important to understand that the propulsion system employs two independently-controlled and operated drive wheels, each being driven by a separate drive-train assembly (i.e. motor-gear box assembly). The suspension arm is pivotally mounted at a single point, between the drive-train assembly and the anti-tip wheel, and spring-biased to a neutral position by a pair of spring-strut assemblies, each one of the pair being disposed on an opposite side of the pivot mount.
In operation, torque from a drive wheel is reacted by the main structural frame resulting in relative rotational displacement between the drive train assembly and the frame. The relative motion therebetween, in turn, effects rotation of the suspension arm about its pivot axis in a clockwise or counterclockwise depending upon the direction of the applied torque. That is, upon an acceleration, or increased torque input (as may be required to overcome or climb an obstacle), counterclockwise rotation of the drive-train assembly will occur effecting upward vertical displacement of the respective anti-tip wheel. Consequently, the anti-tip wheels are “actively” lifted or raised to facilitate such operational modes, e.g., curb climbing. Alternatively, deceleration causes a clockwise rotation of the drive-train assembly, thus effecting a downward vertical displacement of the respective anti-tip wheel. As such, the downward motion of the anti-tip wheel assists to stabilize the wheelchair wheels when traversing downwardly sloping terrain or a negative decline. Here again, the anti-tip system “actively” responds to a change in applied torque to vary the position of the anti-tip wheel.
While the active anti-tip system disclosed in the Schaffner patent '165 offers significant advances by comparison to prior art passive systems, it too has certain drawbacks and limitations. For example, the active anti-tip system of Schaffner, as a practical matter, also requires two spring-strut assemblies to bias the position of each anti-tip wheel. While only requiring a single pivot connection, for mounting or suspending the anti-tip system, the dual spring-strut arrangement is mechanically complex, costly, requires periodic maintenance and adds weight. Yet another disadvantage of such active anti-tip system relates to design limitations caused by the single pivot connection and, consequently, performance compromises. It will be appreciated, for example, that the one piece construction of the suspension arm necessarily requires that both the drive-train assembly and the respective anti-tip wheel must necessarily enscribe the same angle, i.e., the angles are identical. As such, to vary a predefined vertical displacement of the anti-tip wheel, (as maybe desired to overcome larger curbs or obstacles), it is necessary to vary the length of the suspension arm.
One can best appreciate the challenges of this configuration by examining a simple design requirement which will frequently be encountered. Should, for example, a three inch displacement of the forward anti-tip wheel be required to overcome a three inch curb or obstacle, the forward portion of the suspension arm, i.e., from the pivot axis to the anti-tip wheel, would necessarily measure nearly 35 inches to accommodate this design requirement. An assumption is made that drive-train assembly pivots 5° relative to the main structural frame. If, on the other hand, the drive-train assembly were permitted to traverse a larger angle, e.g., 20°, the anti-tip wheels could be positioned significantly farther inboard, to accommodate the 3-inch design requirement. While this approach may enable greater vertical travel of the anti-tip wheel, other wheelchair structure, e.g., a footrest assembly, may interfere and prohibit this design option. It will, therefore, be appreciated that the single pivot mount design, while elegant and simple, leaves few options available for the designer to satisfy other requirements.
Moreover, when altering the horizontal length (in the longitudinal direction) of the suspension arm, the horizontal path taken by the anti-tip wheels will vary in accordance with the arm radius. Stated another way, as the suspension arm varies in length from long to short, the anti-tip wheels traverse a more arcuate path, i.e., rather than a substantially linear path. This variation can significantly impact the curb-climbing ability of the anti-tip system. More specifically, it will be appreciated that when a curb or obstacle impacts the anti-tip wheel at or near a point which is in-line with the wheel's rotational axis, the anti-tip wheel will have a tendency to move upward or downward depending upon the vertical location of the pivot axis of the suspension arm. In a system having a short suspension arm, i.e., one which effects an arcuate travel of the wheel, wherein the wheel axis lies below the pivot axis of the suspension arm, an anti-tip wheel will have a tendency to move downwardly under the above described loading conditions. This downward travel is, of course, contrary to a desired upward motion for climbing curbs or other obstacles.
Finally, inasmuch as powered wheelchairs of this type, i.e., mid-wheeled vehicles, are most appropriately stabilized by a pair of anti-tip wheels disposed forwardly and rearwardly of the main drive wheels, at least one pair of anti-tip wheels is typically castored, i.e., for pivoting/rotation about a vertical axis. Inasmuch as such castored wheels occupy valuable space aboard powered wheelchairs, e.g., interfere with footrest assemblies or an occupants feet/legs, sometimes one of the anti-tip wheel pairs to enable unrestricted yaw control/motion of the wheelchair 2. Consequently, there may be a lag in pitch stabilization response.
A need, therefore, exists for an active anti-tip system, which eliminates the need for multiple strut assemblies, provides greater design flexibility (especially the design flexibility to position the anti-tip wheels at practically any longitudinal and/or vertical position) and facilitates ground contact of the anti-tip wheel system during routine operating conditions.